Re: [hatari-devel] add support for TT ram -> problem with HD emulation and DTA

["Roger Burrows" <rfburrows@xxxxxxxxx>]

On 17 Dec 2014 at 17:48, Nicolas Pomarède wrote:

> Le 17/12/2014 17:29, Roger Burrows a écrit :
>
> > On a TT, to the best of my knowledge, no data is transferred if you try to
> DMA
> > to/from TT RAM.  There is no TT RAM on any other original Atari system, but
> I
> > believe that the same is true for the CT63.  I can easily write a program
> to
> > verify if you wish (I have the hardware).
>
> I thought I read the TT had a 32 bit DMA for the ACSI part, but FDC and
> audio used 2 24 bits DMA ? But maybe I'm mixing with sthg else.
>
> But it could be interesting to see on a TT the different cases of
> reading with the FDC and the HDC to $01080000 for example (depending on
> the expansion RAM you have) and see if at the end of the transfer the
> data are at $80000 (after masking to 24 bits) or $01080000.
>
I've just realised that most of this discussion (including my contributions) is
pointless.  Since the ACSI DMA address is limited to 24 bits, it can't possibly
DMA in TT-RAM.  If the ACSI driver doesn't realise that it's reading/writing
TT-RAM, it will set the DMA address as the low 24 bits of the correct address,
and end up reading/writing the wrong address.

As Cyprian pointed out, the _SCSI_ DMA controller does support the full 32
bits, and therefore it's possible to DMA data to/from TT-RAM directly when
using the SCSI bus.

Roger